Because of large availability of methanol in the industrial world, it has for many years been considered as the most desirable material for obtaining light olefins, aromatic and non-aromatic liquids in the gasoline boiling range. Methanol is also the intermediate of the process starting from synthesis gas. This synthesis gas (a mixture of carbon monoxide and hydrogen) can be derived from natural gases, biomass, coal or heavy oils.
Since light olefins and liquid hydrocarbons (in particular, aromatics) are the most commercially valuable products in the catalytic conversion of methanol and dimethyl ether (dehydration derivative of methanol), high yields in such hydrocarbons are desired in the industrial viewpoint.
Many attempts have been done to find the ideal catalyst. Crystalline aluminosilicate zeolites are the most indicated for this purpose, due mainly to their ordered and molecular-sized framework structure. Among them, the ZSM-5 type zeolites show high performances both in terms of methanol conversion and product selectivities. However, under normal reaction conditions, light paraffin production occurs for more than the third of the hydrocarbon yield as can be seen from Table 1.
TABLE 1 __________________________________________________________________________ The prior art Methanol Reaction conversion Product selectivities (%) Conditions into C.sub.2 -C.sub.4 Non- Ole + Liq T Press. hydrocarbons C.sub.1 -C.sub.4 olefins Aromatics Aromatics (liq = Ole + Catalyst (.degree.C.) (atm) (%).sup.(1) Paraffins (Ole) (Ar) (C.sub.5.sup.+) Ar + C5.sup.+) Ar Ole/Ar __________________________________________________________________________ ZSM-5 zeolite (A) 427 1 99 42 10 41 7 58 51 0.3 427 0.25 99 22 38 20 20 78 58 1.8.sup.(2) 427 0.07 99 15 78 2 5 85 80 39.0.sup.(2) ZSM-5 Zeolite (B) 371 1 99 .sup. 35.sup.(3) .sup. 3.sup.(3) 41 21 65 44 0.1 ZSM-5 Zeolite promoted 385 1 31 5 39 20 36 95 60 1.9 with trimethylphosphite 465 1 88 4 62 8 26 96 70 7.8 (C) 560 1 99 5 66 12 16 94 78 5.5 13X Zeolite bearing 400 1 91 19 81 -- -- 81 81 .infin..sup.(4) 9 Mg and Mn (D) ZSM-5 Zeolite with 400 1 99 (NR) (NR) 37 32 NR NR NR.sup.(5) incorporation of one (31) metal of group I.sub.b, II.sub.a, II.sub.b, III.sub.a, IV.sub.a or VIII (E) __________________________________________________________________________ REMARKS: .sup.(1) Carbon atom basis .sup.(2) Subatmospheric pressure of methanol .sup.(3) reported in Chang C.D. et al J. Cat. 86, 289 (1984) .sup.(4) no liquid hydrocarbons produced .sup.(5) not reported but likely similar to the case of U.S. Pat. No. 3,894,103 PATENTS: (A) U.S. Pat. No. 4,025,575 (Mobil Oil) (B) U.S. Pat. No. 3,894,103 (SiO.sub.2 /Al.sub.2 O.sub.3 = 100) (Mobil Oil) (C) U.S. Pat. No. 3,911,041 (Mobil Oil) (D) Ger. Offen 2,755,229 (Hoeschst A.G.) (E) U.S. Pat. No. 3,894,104 (example 6, Zn) (Mobil Oil).
Modifications of reaction parameters (for example, by using very low-subatmospheric partial pressure of methanol) can decrease the light paraffin production and increase the light olefin yield. However, not only the aromatic formation is also depressed (see Table 1), but also the methanol conversion per pass per unit weight of catalyst is very limited.
Modifications of the catalyst characteristics (for example SiO.sub.2 /Al.sub.2 O.sub.3 ratio) leads to a high liquid hydrocarbon yield with relatively high aromatic content and a high production of light paraffins and a very low production of light olefins (see Table 1).
Modifications of the zeolite chemical composition by incorporating extraneous components lead to one of these two product distributions:
(a) large production of light olefins with significant (or smaller) production of light paraffins, negligible (or much reduced) production of aromatic and non-aromatic liquid hydrocarbons. For this category, we can mention the ZSM-5 zeolite promoted with trimethylphosphite or modified with silica, and the 13X zeolite bearing Mg and Mn (see Table 1). One of the drawbacks of these catalysts is their relatively low methanol conversion within the range of reaction temperature (370.degree.-450.degree. C.); or their very reduced yield in liquid hydrocarbons;
(b) large production of liquid hydrocarbons (with a relatively high aromatic content), and significant production of light paraffins and a very reduced production of light olefins.
For this category, we can mention the case of the ZSM-5 zeolite into which is incorporated Zn or an ion of one among the following groups: I.sub.b, II.sub.a, II.sub.b, III.sub.a, IV.sub.a or VIII (see Table 1).